In upcoming third generation mobile telephony systems, a large part of the load in the networks is expected to be data traffic, caused by e.g. file transfer, web-browsing etc. Furthermore, a large part of the traffic is expected to appear in the downlink direction, and thus, a certain degree of traffic asymmetry must be managed in the networks. The asymmetry can be different in different regions and may also vary with time.
Among the third generation mobile telephony systems, it is claimed that UTRA-TDD efficiently supports asymmetric traffic. However, the support of different degrees of asymmetric traffic in different cells will introduce more interference in the system. Thus, in order to achieve an acceptable trade-off between the requirements set by the traffic demands and the interference, some flexibility must be included in the radio resource management
In a near future, the data traffic in the mobile networks is expected to increase significantly. The load in the networks, according to the argumentation above, is as described above likely to be asymmetric and the degree of asymmetry will vary with time and position within the cellular system.
Thus, a cellular communication system that can allocate a different amount of resources for uplink and downlink transmission would be appreciated, especially if each cell independently can allocate resources for uplink and downlink transmissions according to the traffic demand in each individual cell.
In a system with fixed uplink and downlink allocations, there is no way to adapt the communication resource allocation to the cell-specific traffic demand. On the other hand, in a system where uplink and downlink resources can be exchanged freely, it is in principle possible to adapt the resource allocation to the traffic demand in each cell. An example of such a system is UTRA-TDD, where the communication resource (in this case time slot) allocation can be performed on a cell basis.
For a uniform traffic situation, i.e. a situation where the uplink/downlink asymmetry is the same for essentially the entire system, a resource allocation common to all cells, i.e. global resource allocation, performs well. However, for non-uniform traffic distributions, a global resource allocation performs poorly since it often results in blocking. On the other hand, if the resource allocation is performed totally independently by each cell, and thus adapts to the demands in each cell, the blocking would be minimized.
However, this cell-to-cell independent resource allocation could instead go cause the drawback of increased and unpredictable interference in terms of base-to-base and mobile-to-mobile interference.
In the International patent application WO 00/011888, a system is disclosed, in which the downlink and the uplink fields in each cell are divided into different regions, based on the expected interference in each region. One region is dedicated to uplink traffic, one is dedicated to downlink traffic and one hybrid region has an allocation pattern, which can be changed from time to time. The users are allocated to the different regions according to the quality of the connection. Users with good quality are allocated to a region with relatively high interference and vice versa. Allocating bad links to the dedicated regions while good links are allocated to the hybrid region reduces possible interference.
A problem with the system disclosed in WO 00/01188 is that continuous measurements have to be performed every time when allocation of communication resources to different users takes place. Such an evaluation of the link quality requires both time and computational resources. Since users may move within the cell, the conditions for the links may change with time and frequent reallocations are performed. A high flexibility is achieved but to the price of a large measurement effort and high required computational power.
In U.S. Pat. No. 5,594,720, a cellular communication system is disclosed, in which a frame of slots is divided into two or three regions. When using three regions, two regions are dedicated to uplink and downlink traffic, respectively, and the third region is a hybrid region, here the allocation may vary. The disclosed system is based on directional antennas, and the geometrical pattern of these antennas is used to minimize any co-channel interference.
A problem with the system disclosed in U.S. Pat. No. 5,594,720 is that in order to change the allocation pattern in the hybrid region, information about the cell structures is required. A change of allocation in the hybrid region thus has to be performed in cooperation with neighboring cells, which means that such control has to be performed at a high system level. This results in considerable reporting and signaling activity. The possible flexibility will be reduced significantly in such a system. Furthermore, this solution is only operable in systems using directional antennas. No general solutions for omni-directional antennas are indicated.